Recently, as the portability and cordless tendency of instruments have progressed, a demand for a non-aqueous electrolyte secondary battery such as a lithium secondary battery which is small in size and light in weight and has a high energy density, has been increasingly high. As a positive electrode active material for the non-aqueous electrolyte secondary battery, a composite oxide of lithium and a transition metal such as LiCoO2, LiNiO2, LiNi0.8Co0.2O2, LiMn2O4 or LiMnO2, has been known.
Among them, a lithium secondary battery using a lithium-containing composite oxide (LiCoO2) as a positive electrode active material and using a lithium alloy or carbon such as graphite or carbon fiber as a negative electrode, can obtain a high voltage at a level of 4V, whereby it has been widely used as a battery having a high energy density.
However, in the case of the non-aqueous type secondary battery using LiCoO2 as a positive electrode active material, further improvement of the capacity density per unit volume of a positive electrode layer and the safety, has been desired. On the other hand, there has been a problem of deterioration of the cyclic properties such as gradual reduction of the battery discharge capacity due to repetitive charge and discharge cycles, a problem of the weight capacity density or substantial reduction of the discharge capacity at a low temperature.
In order to solve a part of these problems, it has been proposed in Patent Document 1 that the average particle size of LiCoO2 as a positive electrode active material, be from 3 to 9 μm, the volume occupied by a group of particles having a particle size of from 3 to 15 μm, be at least 75% of the total volume, and the intensity ratio of the diffraction peaks at 2θ=about 19° and 2θ=45° as measured by means of X-ray diffraction using CuKα as a radiation source, be of a specific value, so that it becomes an active material excellent in the coating properties, the self-discharge properties and the cyclic properties. Further, in Patent Document 1, it has been proposed that the positive electrode active material is preferably one which does not substantially have such a particle size distribution that the particle size of LiCoO2 is 1 μm or smaller or 25 μm or larger. With such a positive electrode active material, the coating properties and the cyclic properties have been improved, but, the safety, the volume capacity density and the weight capacity density, have not yet been fully satisfactory.
Further, in order to solve the problem related to the battery characteristics, Patent Document 2 proposes to replace 5 to 35% of Co atoms with W, Mn, Ta, Ti or Nb to improve the cyclic properties. Further, Patent Document 3 proposes to use hexagonal LiCoO2 as a positive electrode active material to improve the cyclic properties, wherein the c axis length of the lattice constant is at most 14.051 Å, and the crystal lattice size of (110) direction of the crystal lattice is from 45 to 100 nm.
Further, Patent Document 4 proposes that a lithium composite oxide of the formula LixNi1-mNmO2 (wherein 0<x<1.1, 0≦m≦1), of which the primary particles are plate-like or columnar, the ratio of (volume standard cumulative 95% size−volume standard cumulative 5% size)/(volume standard cumulative 5% size) is at most 3, and further, the average particle size is from 1 to 50 μm, has a high initial discharge capacity per weight and further is excellent in the charge and discharge cyclic durability.
Further, Patent Document 5 proposes to lithiate a cobalt compound powder in the form of secondary particles with an average particle size of 0.5 to 30 μm formed by agglomeration of primary particles of cobalt hydroxide, cobalt oxyhydroxide or cobalt oxide with an average particle size of from 0.01 to 2 μm. However, also in this case, it is not possible to obtain a positive electrode material having a high volume capacity density, and further, the material is insufficient also with respect to the cyclic properties, the safety or the large current discharge properties.
Patent Document 6 and Patent Document 7 propose a method of covering lithium cobalt oxide particles with a different metal element by a sol-gel process, but the covered lithium cobalt oxide is unsatisfactory in the battery performance i.e. the discharge capacity, the charge and discharge cyclic durability and the safety. Further, although an alkoxide of the different metal element as a starting material may be suitable at the laboratory level, it is too expensive to employ industrially. Further, as the alkoxide is very sensitive to water and is likely to be hydrolyzed, such a reaction apparatus that the alkoxide will not be influenced by water in the air will be required, and the cost of equipment tends to be high, thus raising the cost, and such is problematic economically.
Further, Patent Document 8 proposes to react a colloidal coating liquid obtained by adding water to (NH4)2HPO4 and Al(NO3)3.3H2O with lithium cobalt oxide particles. However, the covered lithium cobalt oxide is unsatisfactory in the battery performance i.e. the discharge capacity, the charge and discharge cyclic durability and the safety.
As described above, in the prior art, with respect to a lithium secondary battery employing a lithium composite oxide as a positive electrode active material, it has not yet been possible to obtain one which sufficiently satisfies all of the volume capacity density, the safety, the coating uniformity, the cyclic properties and further the low temperature characteristics.
Patent Document 1: JP-A-6-243897
Patent Document 2: JP-A-3-201368
Patent Document 3: JP-A-10-312805
Patent Document 4: JP-A-10-72219
Patent Document 5: JP-A-2002-60225
Patent Document 6: JP-A-2000-306584
Patent Document 7: JP-A-2002-279991
Patent Document 8: JP-A-2003-7299